Problem 17 - Chemistry The Central Science In Si Units

Question

Ignoring protolysis reactions (i.e. proton transfer reaction), specify what ions are present in a solution upon dissolving each of the following substances in water: \((\mathbf{a}) \mathrm{L}_{2} \mathrm{CO}_{3},\) (b) \((\mathrm{NH} 4)_{3} \mathrm{PO}_{4}\) (c) \(\mathrm{Na}_{2} \mathrm{Cr}_{2} \mathrm{O}_{7}\) (d) \(\mathrm{NaPF}_{6^{*}}\)

Step-by-Step Solution

Answer

In solutions: (a) \( 2\mathrm{Li}^+ \), \( \mathrm{CO}_3^{2-} \); (b) \( 3\mathrm{NH}_4^+ \), \( \mathrm{PO}_4^{3-} \); (c) \( 2\mathrm{Na}^+ \), \( \mathrm{Cr}_2\mathrm{O}_7^{2-} \); (d) \( \mathrm{Na}^+ \), \( \mathrm{PF}_6^- \).

Step 1: Dissolve Li2CO3

Upon dissolving lithium carbonate, \( \mathrm{Li}_2\mathrm{CO}_3 \), in water, it dissociates into its constituent ions. Lithium carbonate is composed of lithium ions \( \mathrm{Li}^+ \) and carbonate ions \( \mathrm{CO}_3^{2-} \). Thus, the ions present in the solution are \( 2\mathrm{Li}^+ \) and \( \mathrm{CO}_3^{2-} \).

Step 2: Dissolve (NH4)3PO4

Ammonium phosphate, \( (\mathrm{NH}_4)_3\mathrm{PO}_4 \), dissociates in water into ammonium ions \( \mathrm{NH}_4^+ \) and phosphate ions \( \mathrm{PO}_4^{3-} \). Therefore, the ions present in the solution are \( 3 \mathrm{NH}_4^+ \) and \( \mathrm{PO}_4^{3-} \).

Step 3: Dissolve Na2Cr2O7

Sodium dichromate, \( \mathrm{Na}_2\mathrm{Cr}_2\mathrm{O}_7 \), dissolves to form sodium ions \( \mathrm{Na}^+ \) and dichromate ions \( \mathrm{Cr}_2\mathrm{O}_7^{2-} \). Hence, the ions present in the solution are \( 2\mathrm{Na}^+ \) and \( \mathrm{Cr}_2\mathrm{O}_7^{2-} \).

Step 4: Dissolve NaPF6

Sodium hexafluorophosphate, \( \mathrm{NaPF}_6 \), dissociates into sodium ions \( \mathrm{Na}^+ \) and hexafluorophosphate ions \( \mathrm{PF}_6^- \). Thus, the ions present in the solution are \( \mathrm{Na}^+ \) and \( \mathrm{PF}_6^- \).

Key Concepts

Ionic Compounds

Ionic compounds are formed by the chemical bonding of metal and non-metal ions. In these compounds, metals, typically located on the left side of the periodic table, lose electrons to form positive ions or cations. Non-metals, which are usually found on the right side of the periodic table, gain electrons to form negative ions or anions. When these ions are bonded together, they create a stable compound through ionic bonds.

• Example: Lithium carbonate (\(\mathrm{Li}_2\mathrm{CO}_3\)) consists of lithium ions (\(\mathrm{Li}^+\)) and carbonate ions (\(\mathrm{CO}_3^{2-}\)).
• In \(\mathrm{(NH}_4)_3\mathrm{PO}_4\) (ammonium phosphate), ammonium ions (\(\mathrm{NH}_4^+\)) bond with phosphate ions (\(\mathrm{PO}_4^{3-}\)).
• Sodium dichromate (\(\mathrm{Na}_2\mathrm{Cr}_2\mathrm{O}_7\)) contains sodium ions (\(\mathrm{Na}^+\)) and dichromate ions (\(\mathrm{Cr}_2\mathrm{O}_7^{2-}\)).

Understanding the formation and dissociation of ionic compounds is crucial in analyzing solutions and chemical reactions.

Aqueous Solutions

An aqueous solution is a type of solution in which water is the solvent. Many ionic compounds can dissolve in water, a property that results from water's ability to stabilize the ions after they dissociate from the solid compound. This occurs because water molecules are polar, meaning they have a partial positive charge on one side and a partial negative charge on the other. This allows them to surround and interact with various ions effectively.

• For example, when \(\mathrm{Li}_2\mathrm{CO}_3\) dissolves, the water separates \(\mathrm{Li}^+\) and \(\mathrm{CO}_3^{2-}\) ions.
• In the case of \((\mathrm{NH}_4)_3\mathrm{PO}_4\), water separates \(3\mathrm{NH}_4^+\) ions and one \(\mathrm{PO}_4^{3-}\) ion.

This is especially important in chemistry when determining the behavior of substances in water or predicting the outcome of reactions.
Knowing which substances form aqueous solutions and how they dissociate can play a significant role in fields like environmental science and medicine.

Ion Identification

Ion identification involves recognizing the specific ions produced when ionic compounds dissolve in solvents like water. Identifying these ions is crucial because they determine the chemical properties and reactions that can occur.Steps for Identifying Ions:

• Identify the formula of the ionic compound, such as \(\mathrm{Li}_2\mathrm{CO}_3\).
• Determine the ions that make up the compound. This means splitting the compound into its cation (positive ion) and anion (negative ion).
• From \(\mathrm{Li}_2\mathrm{CO}_3\), splitting gives us \(\mathrm{Li}^+\) and \(\mathrm{CO}_3^{2-}\).

By understanding what ions are present, you can predict reactions and behaviors in an aqueous solution. For contact with certain compounds, identifying the ions makes predicting pH changes, solution conductivity, and precipitation reactions possible. This is an essential skill in both academic and applied chemistry.